Lighting system for transmitting and releasing luminescent radiation

ABSTRACT

An optical conduit ( 53 ) for transmitting and releasing luminescent radiation ( 61, 62, 63 ) emitted from a luminescent concentrator ( 51 ), the luminescent concentrator ( 51 ) and optical conduit ( 52 ) comprising an optical system which is adapted to release to an area to be illuminated the luminescent radiation which is otherwise trapped in the optical system by total internal reflection. The optical conduit ( 53 ) includes a luminaire ( 57 ), such as a plurality of scattering regions for scattering trapped light so that the scattered light acquires the angle of incidence required to released from the conduit ( 53 ). The scattering regions may be provided by shape irregularities on the surface of the conduit ( 53 ) or inhomogeneities within the conduit ( 53 ). The conduit ( 53 ) may consist of flexible overlaid light guides ( 65   a,    65   b  and  65   c ) and a luminaire fitting ( 55 ) optically coupled thereto.

FIELD OF INVENTION

The present invention relates to a lighting system for transmitting andreleasing luminescent radiation from luminescent concentrators orcollectors and, in particular, to a lighting system in which trappedlight generated from a luminescent solar concentrator may be transmittedand released.

Such lighting systems have particular application as means forsubstantially increasing the useful light provided by a luminescentconcentrator whose output is transmitted to the interior of a buildingby a clear, smooth, optical conduit. In particular, the invention seeksto provide a means by which light that is ordinarily trapped in theconcentrator can travel down the conduit, and a means by which thislight can be released from the conduit at a location where illuminationis required.

BACKGROUND ART

It is a fundamental principle of linear optics that no system in whichall the light enters from outside the system can ever have any trappedlight. It is the common general knowledge of persons skilled in the artthat light trapping requires that once light is generated within asystem it must be totally internally reflected off all surfaces and thatthe basic reason why linear optical systems cannot trap light that hasentered from the outside is the fact that if light passes in through asurface at a particular angle then light travelling in the exactopposite direction will pass out through that surface. This is becauselinear optical systems are time reversible (Pedrotti, F. L., andPedrotti, L. S., Introduction to Optics, page 38. Practice HallInternational, Inc, New Jersey, 1987.).

If light enters a light guide system from the outside and is reflectedfrom the far end, it will tend to reach the entry surface with an anglesimilar to that with which it started, and will thus be able to passthrough that surface and so escape from the system. This means that thesystem in which light enters from an external source can have no trappedlight.

Only light sources that generate light inside a material can produceinternally trapped light (Saleh, B. E. A. and Teich, M.C., Fundamentalsof Photonics, pages 18, 39, and section 16.1. Wiley, 1991). Examples ofinternal light sources are fluorescent molecules and electrons and holepairs in semiconductor light emitting diodes. A portion of the emissionfrom such sources is often completely trapped within the system by totalinternal reflection (Saleh and Teich 1991).

The key point is that this light can travel on a closed path but such apath cannot be duplicated by light entering from the outside. Lightentering from the outside and returning to the entry surface will alwaysstrike it at an angle that lies outside the total internal reflectioncone of angles and thus exits from the system.

Another simple argument based on fundamental thermodynamics can be usedto show that no type of linear optical system can have trapped lightenter from the outside. If it were possible for a linear system tolosslessly accumulate light produced externally, then the light would becontinuously accumulated without limit and so the energy density insidewould increase without limit. Such a system could be used to generatetemperatures which were arbitrarily larger than the source temperature—aclear violation of the Second Law of Thermodynamics. Internal sourcesystems do not suffer from this problem because the accumulation oftrapped light changes the properties of the system in a way thatprevents the endless accumulation of light. For example, fluorescentdyes become opaque to their own emissions at large enough photondensities due to non linear effects and so a system using these dyeswill eventually cease to accumulate trapped light.

Luminescent solar concentrators (also called light receiving stacks) areof increasing interest because of their ability to contribute to thetransmission of sunlight to the interior of buildings, owing in largepart to their lower installation, running and maintenance costs overboth conventional lighting systems and solar lighting systems that usetracking mirrors.

Luminescent solar concentrators contain at least one luminescent speciescapable of emitting luminescent radiation upon excitation by incidentsolar radiation. A large proportion of the emitted luminescent radiationis totally internally reflected by the surfaces of the medium from whichthe concentrator is fabricated and propagates inside the medium to theconcentrator's end surfaces.

For example, in a luminescent solar concentrator comprising a flatrectangular sheet, light emitted by luminescent species at small anglesto the planar axis of the sheet is totally internally reflected by thesheet's upper, lower and side surfaces and propagates to one end surfacewhere it can escape. It is also apparent that light emitted byluminescent species nearly perpendicular to the sheet's planar axisquickly escapes through an upper, lower or side surface withoutundergoing total internal reflection.

However, some of the luminescent light emitted at intermediate angles tothe sheet's planar axis is totally internally reflected by the sheet'supper, lower and side surfaces and propagates to one end surface wheretotal internal reflection from the end surface causes it to reverse itspath and be reflected back down the sheet. This light is completelytrapped within the sheet and is unable to escape through any smoothsurface of the sheet. For example, in a flat rectangular sheet ofrefractive index 1.5 surrounded on all sides by air, each of the sheet'ssix surfaces release approximately 12.7% of the luminescent radiation,and 23.6% of the luminescent radiation is trapped within the sheet.(Most of the trapped light is eventually removed by absorption by theluminescent molecular species or by scattering from defects.)

The prior art has not successfully provided a means by which thistrapped light may be released from the conduit at a location whereillumination is required.

Luminescent concentrator/conduit systems known in the prior art consistof a luminescent concentrator which is connected to a smooth,transparent optical conduit which is, in turn, connected to a luminaire(which may be no more than the end of the optical conduit). Luminescentradiation from the concentrator enters the conduit where it ischannelled by means of total internal reflection to the luminaire whichallows the light to escape from the system in the requireddirections(s). For efficient light transfer to occur from theconcentrator to the conduit, and along the conduit, the cross sectionalarea of the conduit (which may change along its length) must never besmaller than the exit area of the concentrator.

It has been found by the present inventors that if the joint between theconcentrator and the conduit has a mismatch in refractive indices (aswill always occur with an air gap and may occur with some glued joints),then a substantial fraction of the luminescent radiation striking thejoint is reflected away from the conduit, back into the concentrator.For many concentrator geometries, this light is unable to escape throughany surface.

It is therefore an object of the present invention to provide an opticalconduit that includes luminaire means through which such ‘trapped light’can exit the system in a useful manner. It is another object of thepresent invention to ensure that the luminescent concentrator andconduit are sufficiently closely coupled to enable the concentrator's‘trapped light’ to enter the optical conduit, where it willsubstantially increase the amount of light that reaches the luminairemeans at the end of the conduit.

SUMMARY OF INVENTION

According to the present invention there is provided a lighting systemfor enabling release of trapped light therefrom as useful illumination,comprising a luminescent concentrator, an optical conduit opticallycoupled to the luminescent concentrator, and a luminaire means, thelighting system being fabricated of light propagating material andhaving surfaces which define an optically continuous solid opticalsystem for enabling light to propagate therethrough by total internalreflection off the surfaces, wherein the said light is luminescentradiation emitted from the luminescent concentrator and wherein aportion of the said luminescent radiation would be, if it were not forthe luminaire means, trapped in the optical system and unable to bereleased therefrom, the luminaire means comprising at least one regionbeing so arranged as to allow the said otherwise trapped portion of theluminescent radiation to acquire an angle of incidence to the or eachsaid region that will enable release of the said otherwise trappedportion of the luminescent radiation from the lighting system, wherebythe quantity of light released by the lighting system as usefulillumination is enhanced.

Preferably, the luminaire means comprises a plurality of scatteringregions for scattering the trapped portion of the luminescent radiationso that the scattered radiation acquires the said angle of incidence.

In a preferred form of the invention, the optical conduit is prepared byfirstly extruding or casting a sheet of polymer material, then cuttingto size and suitably polishing the edges.

Preferably, the plurality of scattering regions comprise shapevariations or irregularities on the surface of the optical conduit atspecific locations and of a predetermined spatial extent. The surfaceshape variations may comprise non-flat surfaces made by externalabrasion, texturing, moulding or chemical etching.

For instance, the surface shape variations of the optical conduit maycomprise a surface roughened by sand paper.

The plurality of scattering regions may also comprise a surface coatingon the optical conduit, wherein the surface coating includes particulatematter capable of scattering the otherwise trapped portion of theluminescent radiation.

In another embodiment of the invention, the plurality of scatteringregions comprise particulate matter embedded within the optical conduitor inhomogeneities within the optical conduit.

The purpose of the scattering centres is to scatter the trapped lightout of the conduit which would otherwise remain trapped in the lightingsystem.

The plurality of scattering regions function by creating a change in theangle of incidence of the trapped light with respect to a surface of theoptical conduit so that the light is emitted or released through thatsurface.

In yet another embodiment of the invention, the luminaire meanscomprises a portion of the conduit which is expanded greatly in crosssectional area so that the otherwise trapped portion of the luminescentradiation strikes a surface of the expanded portion at an angle thatpermits transmission through that surface. The luminaire means may, insuch an embodiment, be cast simultaneously with the remainder of theconduit.

In still another embodiment, the luminaire means may be joined to theconduit by an optical joint. For most efficient results, the luminescentconcentrator is coupled to the optical conduit, and the conduit iscoupled to the luminaire means, by optical joints at which there isideally no mismatch in refractive index between the concentrator, jointmaterial, conduit and luminaire means.

Preferably, the optical joint is provided by a transparent couplingagent with a refractive index as close as possible to the square root ofthe product of the refractive indices of (a) the concentrator and theconduit and (b) the conduit and luminaire means (ie the geometric meanof their refractive indices).

UV cured optical cements or optical grade epoxy glues are suitablecoupling agents. Another suitable coupling agent is optical gel,although, if this is used, the optical conduit must be held inmechanical alignment by other means.

It may also be possible to couple the conduit to the concentrator andthe luminaire means to the conduit by other techniques known in the art,such as by solvent welding, ultrasound welding, and the like.

It is possible to eliminate the need for a coupling agent between theconcentrator and conduit, by coating the luminescent material onto partof a continuous optical conduit or by casting the concentrator as acontinuation of a preformed optical conduit, or by casting the opticalconduit as a continuation of a preformed concentrator. Alternatively,the concentrator and optical conduit may be cast simultaneously.

However the joint from the luminescent concentrator to the opticalconduit is made, it should ideally be defect free with no bubbles orvoids and there should be no surplus coupling agent on the surfaces nearthe joint so that the lighting system is as optically continuous aspossible and so that light may freely pass from the concentrator to theoptical conduit without reflection or scattering. This opticalcontinuity enables the ‘trapped light’ (ie the light that would betrapped in the absence of optical continuity) to enter the opticalconduit, whereas a simple alignment or butt joint, even with very smoothsurfaces, would not.

Preferably, the luminaire means comprises a light scattering portion ata first of two opposed ends of a light fitting adapted to be located inan area to be illuminated, the light fitting being optically coupled tothe conduit at the second of its opposed ends, the light scatteringportion having been treated in such a way so as to enable the otherwisetrapped portion of the luminescent radiation to be released therefrom.

In a still further embodiment of the invention, the luminaire means mayform a terminal part of the optical conduit.

Preferably, the luminescent concentrator is illuminated with sunlight.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be readily understood and put intopractical effect, reference will be made to the accompanying drawings,in which:

FIG. 1 is a schematic representation of the path of light emitted byluminescent species at small and large angles to the planar axis of aflat rectangular sheet used as a luminescent solar concentrator,

FIG. 2 is a schematic representation of the path of light emitted byluminescent species at an intermediate angle to the planar axis of thesheet shown in FIG. 1,

FIG. 3 is a schematic representation of a luminescent solarconcentrator/conduit system known in the prior art,

FIG. 4 is a schematic representation of a luminescent solarconcentrator/conduit system which includes an optical conduit accordingto a first preferred embodiment of the invention,

FIG. 5 is a schematic cross-sectional representation of an end portionof an optical conduit according to a second preferred embodiment of theinvention,

FIG. 6 is a schematic representation of a luminescent solarconcentrator/conduit system which includes the optical conduit to FIG.4, and

FIG. 7 is an isometric view of a luminescent solar concentrator/conduitsystem according to another preferred embodiment of the invention.

FIG. 8 is an isometric view of an alternate luminaire means to be usedin the luminescent solar concentrator/conduit system shown in FIG. 7.

MODES FOR CARRYING OUT THE INVENTION

In the luminescent solar concentrator sheet 11 shown in FIG. 1, light 12which is emitted by luminescent species (shown as the origin of thearrows) at small angles to the planar axis of the sheet 11 is totallyinternally reflected by the sheet's smooth lower surface 13 and smoothupper surface 14 and propagates to the end surface 15 where its angle ofincidence and the refractive index at the interface enables it to bereleased or to escape from the sheet 11. Light 16 which is emitted byluminescent species nearly perpendicular to the planar axis of the sheet11 immediately escapes through the upper surface 11 without undergoingtotal internal reflection.

In the luminescent solar concentrator sheet 18 shown in FIG. 2, light 19which is emitted by luminescent species at intermediate angles to theplanar axis of the sheet 18 is totally internally reflected by thesheet's smooth lower surface 20 and smooth upper surface 21 andpropagates to the smooth end surface 22 where its angle of incidence andthe refractive index at the interface causes it to be totally internallyreflected. The general direction of the path of the light 19 is nowreversed by the total internal reflection and the light 19 is reflectedback down the sheet 18. This light 19 is completely trapped within thesheet's smooth surfaces.

The traditional luminescent concentrator conduit system 23 shown in FIG.3 comprises a luminescent solar concentrator 24 connected to a smoothlysurfaced transparent optical conduit 25 which is, in turn, connected toa cupped luminaire 26. The system 23 is surrounded by air. Thecross-sectional area of the conduit 25 is, in this embodiment, the sameas the cross-sectional exit area 27 of the concentrator 24, and there isan optical joint 28 between the concentrator 24 and conduit 25 (wherebythe refractive indices (RI's) of the concentrator 24, joint 28 andconduit 25 are identical), thereby enabling efficient light transferfrom the concentrator 24 to the conduit 25 and along the conduit 25.However, not all of this light is able to escape through the end surface29 so that the cupped luminaire 26 may direct the released light asshown in FIG. 3.

If the luminescent concentrator/conduit system 23 has an RI of 1.5, eachof the six surfaces of the system 23 will release about 12.7% of theluminescent radiation, whereas about 23.6% of the luminescent radiationwill be trapped within the system 23, most of this trapped light beingeventually absorbed by the luminescent species in the concentrator 24 orbeing scattered by defects.

The effect of the cupped luminaire 26 as a means for directing lightconcentrated by the system 23 is, therefore, not significant, as it isonly able to direct light that has been released through the end surface29, and much useful light is either lost through the other surfaces ortrapped within the system 23.

The luminescent concentrator/conduit system 30 shown in FIG. 4 comprisesa luminescent solar concentrator 31 connected by an optically continuousjoint 32 to a smooth optical conduit 33. A luminaire 34 for the conduit33 is produced by introducing light scattering centres at theappropriate portion of the conduit 33 where illumination is required. Inthe present embodiment, the scattering centres are on the surface of theconduit 33, but they may be in the bulk material from which the conduit33 is fabricated. The scattering centres scatter the trapped light outof the conduit 33 by creating a change in the angle of incidence of thetrapped light with respect to the surface portions. Such scatteringcentres serve as the luminaire 34.

FIG. 5 shows an enlarged end portion 35 of an optical conduit 36. Theenlarged end portion 35 is optically continuous with the conduit 36 andhas a greatly enlarged cross-sectional area so as to enable the totallyinternally reflected light 37 to strike a surface of the end portion 35at an angle that permits the light to be released through that surface.The optical continuity is provided by an optical joint between theseparately cast conduit 36 and the enlarged end portion 35, or bycasting the optical conduit simultaneously with the enlarged headportion 35. The shapes which may be suitable for the enlarged headportion 35 will be described later in the specification.

As shown in FIG. 6, a concentrator sheet 40 dyed with about 70 ppmLumogen 083™ “yellow” dye (which emits green light at the concentrationsused) was exposed to a fluorescent lamp 41 only at one end, as shown,with the middle portion 44 of the sheet 40 serving as an optical conduit42 as it did not have any light exposure thereon. The total light outputfrom the optical conduit 42 at the opposite end was measured with anintegrating sphere 43. No optical joint was considered necessary forthis example of a concentrator/conduit system.

The sheet dimensions were 270 mm×20 mm×2 mm.

The final 50 mm of the optical conduit 42 was treated in various ways sothat the total internally reflected light 45 could be scattered andreleased out the side.

As will be described later, it was found that light was released fromthe conduit 42 both through the end surface 46, hereinafter defined asend light, and through the side surfaces at the end of the conduit 42(top and bottom side surfaces 47 and 48 shown, but near side and farside surfaces not shown), hereinafter defined as side light.

Various treatments were found suitable for the final 50 mm of theoptical conduit 42 including (a) roughening one or more surfaces with1200 grade, 600 grade, 400 grade, 240 grade and 120 grade “wet and dry”sand papers (b) attaching “diffuse” sticky tape to the top and bottomside surfaces (c) gluing diffuser sheets to the surfaces and (d) dippingthe final 50 mm into acetone for various intervals (which roughens thesurfaces). The grooves made with sand papers were mostly perpendicularto the long axis of the conduit 42 and their direction seemed not to beimportant. These various treatments have different efficiencies.

All of the above treatments gave more light output than no treatment,where the only light output was through the end surface 46 as end light.The best results of 63% more light were obtained when 1200 grade paperwas used to roughen the top side surface only. However, even 120 gradepaper on all side surfaces gave 43% more light than no treatment. In theconduit treated with 1200 grade paper, the side light leaked out overthe first 3 or 4 centimetres of the 50 mm treated length. In the moreroughly treated conduits, the side light leaked out within the firstcentimetre of the 50 mm treated length of conduit. However, in all ofthe conduits treated with sand paper, about one quarter of the lightcame out the end surface 46. This suggests that the surface roughnesswas imperfect, as ideally almost all the light should have been releasedas side light if the surface was sufficiently rough.

In experiments with various conduits, the level of trapped light gainshas also been sensitive to the quality of the joint and of theluminescent concentrator. Gains in excess of 50% are practical.

The extent to which the side surfaces of the conduit should be roughenedwith sand paper or other forms of surface abrasion must not be such thatit will cause a reversal in the direction of the internally scatteredlight, and nor should the length of the roughened region be so shortthat any reversely scattered light cannot undergo a second or moresubsequent forward scattering.

Some treatments have been found to backscatter both trapped and endlight, and these treatments must be avoided.

It is envisaged that an even more improved light output may be achievedwhen bulk scattering regions are present on or within the opticalconduit.

For instance, during the manufacture of the optical conduit, bulkscattering centre forming materials, such as calcium carbonate, zeolitesand titanium dioxide, may be included in the medium that is shaped andsolidified into the conduit. Alternatively, these materials may beincluded in a paint or other surface applicable material that is coatedon to the conduit. A coating of a polymer that includes fine scatteringparticles may also be used.

Small bubbles or other inhomogeneities may also be incorporated in theconduit during its manufacture to generate scattering regions. Suchinhomogeneities may be produced by adding particles of a polymer orother scattering material which has a slightly different RI to the othermaterial from which the conduit is made. The region of conduit at whichsuch inhomogeneities or small bubbles occur serves as the luminairemeans.

The above treatments may result in an increase in the frequency ofscattering interactions within the conduit, so that otherwise trappedlight can be scattered such that it acquires an angle of incidence tothe side surfaces for the scattered light to escape the conduit.Preferably, the increased scattering is in a forward direction along theconduit.

FIG. 7 shows a luminescent concentrator conduit system 50 comprising athree layered stack solar collector or concentrator 51 coupled by anoptical joint 52 to a flexible optical conduit 53 which comprises threeoverlaid light guides 65 a, 65 b and 65 c and a luminaire fitting 55.The solar collector 51 (consisting of three overlaid fluorescent sheets51 a, 51 b and 51 c) is located externally of a building or the like sothat it is exposed to sunlight (shown impacting the solar collector 51by arrow 57 and being absorbed by luminescent species at locations 58,59 and 60, so that these species fluorescently emit light (luminescentradiation) shown by arrows 61, 62 and 63 that is trapped within thesolar collector 51 by total internal reflection). The optical cable 53passes through a wall 54 of the said building to the area to beilluminated.

The aforementioned system 50 is similar to a sunlight collecting andtransmitting system disclosed in Australian Patent No. 661,716 to thesame inventors. The teachings of Australian Patent No. 661,716 areincluded herein by reference.

The specially fabricated luminaire fitting 55, adapted to be located inthe area to be illuminated, is coupled to the end surface of the threeoverlaid light guides 65 a, 65 b and 65 c by an optical joint 56. At thefree end of the fitting 55 is a terminal scattering portion 57 which istreated in any of the aforementioned ways so that the portion 57 canserve as a luminaire for the trapped light to exit the system (as shownby the arrows which radiate outwardly from all side surfaces of theportion 57).

Alternatively, the terminal scattering portion may comprise the entireluminaire fitting 55 so that the flexible overlaid light guides areoptically coupled directly to a terminal scattering member. Such ascattering member may be made of diffuse material, such as opalescentplastic, or include an outer layer of diffuse material. The diffusematerial is envisaged to cause a gradual scattering of light in theforward direction.

As previously mentioned with reference to FIG. 5, the conduit may alsoterminate in an enlarged end portion 35, such as a cone shaped memberwith a curved end surface (where the conduit is cylindrical) or as anangular sector of a cylinder member (where the conduit is a rectangularprism) of the same refractive index as the conduit but, say, about fivetimes the thickness of the conduit. The angular sector of a cylindermember 66 is shown in FIG. 8 coupled by an optical joint 67 to aflexible rectangular prism conduit 68 consisting of overlaid lightguides similar to that shown in FIG. 7. As a result of the enlargedconfiguration of the end portion, which serves as the luminaire means,previously trapped light will pass from the conduit into the enlargedend portion and escape out of the end surface of the enlarged endportion to illuminate the adjacent area. The side surfaces 70 and 71 ofthe enlarged end portion 35 or 66 may include a plurality of scatteringregions such as surface coatings, or shape variations formed as a resultof the casting process or abrasion to assist in release of trappedlight. The surface coating includes particulate matter capable ofscattering trapped light.

Various other modifications may be made in details of design andconstruction without departing from the scope and ambit of theinvention.

What is claim is:
 1. A lighting system which utilizes fluorescentspecies to generate fluorescently emitted radiation within a luminescentconcentrator, said lighting system comprising: a luminescentconcentrator containing fluorescent species, an optical conduitoptically continuous with the luminescent concentrator, and a luminairemeans, optically continuous with said optical conduit, said luminairemeans being adapted to be located in an area to be illuminated, whereinlight is provided by the fluorescent radiation emitted within saidluminescent concentrator, the lighting system being fabricated of lightpropagating material and having surfaces which define an opticallycontinuous solid optical system for enabling said light to propagatetherethrough by total internal reflection off the surfaces, and whereinsaid luminaire means contains at least one region allowing an internallytrapped portion of the fluorescent radiation within the lighting systemto acquire an angle of incidence to a surface of the luminaire meansthat enables release of said internally trapped portion of thefluorescent radiation from the luminaire means.
 2. The lighting systemof claim 1, wherein the luminaire means comprises a plurality ofscattering regions for scattering the trapped portion of the fluorescentradiation so that the scattered radiation acquires the said angle ofincidence.
 3. The lighting system of claim 2 wherein the plurality ofscattering regions comprise shape variations or irregularities on thesurface of the optical conduit at specific locations and of apredetermined spatial extent.
 4. The lighting system of claim 3 whereinthe surface shape variations of the optical conduit comprise a sandpaper roughened surface.
 5. The lighting system of claim 2 wherein theplurality of scattering regions comprise a surface coating on theoptical conduit, wherein the surface coating includes particulate mattercapable of scattering the otherwise trapped portion of the luminescentradiation.
 6. The lighting system of claim 2 wherein the plurality ofscattering regions comprise inhomogeneities within the optical conduit.7. The lighting system of claim 6 wherein the inhomogeneities compriseparticulate matter embedded within the optical conduit.
 8. The lightingsystem of claim 2 wherein the plurality of scattering regions compriseat least one diffuse self adhesive film attached to the optical conduitor diffuser sheets glued to the optical conduit.
 9. The lighting systemof claim 1, wherein the luminaire means comprises a portion of theoptical conduit which is expanded greatly in cross sectional area sothat the otherwise trapped portion of the fluorescent radiation strikesa surface of the expanded portion at an angle that permits transmissionthrough that surface.
 10. The lighting system of claim 9, wherein theexpanded portion of the optical conduit includes a plurality ofscattering regions comprising shape variations on one or more surface ofthe expanded portion, or inhomogeneities within the expanded portion, ora surface coating including particulate matter capable of scatteringlight on one or more surface of the expanded portion.
 11. The lightingsystem of claim 1, wherein the luminaire means comprises a lightscattering region at a first of two opposed ends of a light fittingadapted to be located in an area to be illuminated, the light fittingbeing optically coupled to the conduit at the second of its opposedends, the light scattering region having been treated in such a way soas to enable the otherwise trapped portion of the fluorescent radiationto be released therefrom.
 12. The lighting system of claim 1 wherein theoptical conduit comprises an extruded or cast sheet of polymer material,which sheet has polished edges.
 13. The lighting system of claim 3,wherein the surface shape variations comprise non-flat surfaces selectedfrom the group consisting of externally abraded, textured, moulded orchemically etched surfaces.